A typical offshore terminal for mooring a ship while transferring oil or gas to or from the ship, includes a buoy connected by a hawser to the ship and a fluid conduit which is also connected to the ship. The ship must be permitted to drift about the buoy, so that a fluid swivel is required along the fluid conduit which connects a stationary pipeline at the seabed to the rotating vessel. A floating hose structure is often utilized to connect to the vessel, since this facilitates pickup of the hose structure and can minimize stresses on it.
In one type of terminal, commonly referred to as the CALM type (catenary anchor leg mooring), the buoy is anchored by several catenary chains to the seabed, and the buoy moves up and down with the tide and waves. The inner or buoy end of the floating hose structure is subjected to some wave action, but the effect is minimized by the fact that the inner end of the hose structure will move up and down as the buoy moves up and down in the waves.
In another type of terminal, commonly referred to as the SALM type (single anchor leg mooring), the buoy is anchored by a single leg or riser to the seabed, such as a column or single chain. The anchor leg is short enough that it is always held in tension by the buoy, and consequently, the buoy does not move up and down with the waves. If a fluid swivel were to lie at the top of such a buoy, and if the buoy end of a floating hose structure were to extend all the way to the fluid swivel or to a short pipe projecting therefrom, then the buoy end of the hose structure could be subjected to heavy pounding from wave action. That is, since the buoy does not move up and down with the waves, the upper portion of the buoy and any hoses connected thereat would be repeatedly uncovered and covered by the waves and subjected to pounding from the waves. To prevent wave damage, SALM (single anchor leg mooring) terminals typically utilize fluid swivels located below the buoy, such as at the sea base, so that the inner end of the floating hose structure is below the level of waves. However, this results in the fluid swivel being relatively inaccessible. A fluid swivel is considered a high maintenance item, in that leakage occasionally occurs and its seals then must be replaced. However, if the fluid swivel is located underwater, then the terminal usually must be dismantled to remove the fluid swivel for maintenance, which results in considerable cost. Also, if the anchor leg passes through the vertical axis of the rotation of the fluid swivel, then the anchor leg may have to be detached from the seabed base to remove the fluid swivel, unless somewhat costly bypassing mechanisms are resorted to.